When looking for a suitable capacitor for use in a magneto ignition system, don’t expect to see a data sheet claiming ‘suitable for use in magnetos’ - if only it were that easy! Instead, it is necessary to consider the three main properties of a capacitor which are important in this application.
1: Capacitance. The correct capacitance is important. If it is too large, then the rate of change of the primary current when the points open will be slowed down, reducing the maximum voltage produced in the secondary winding. This lower secondary voltage may then be too low to achieve a spark at the spark plug. If the capacitance is too small, then arcing will occur at the contact breaker points. Current flow across the points will carry contact material (platinum in early magnetos or tungsten in later ones) from one contact to the other. On a single cylinder magneto, primary current flow will always be in the same direction every time the points open so if the contacts show signs of pitting (a tip on one side and a corresponding pit on the other), the capacitance value needs to be adjusted. If the contact material transfers from the negative to the positive point, the condenser capacitance should be increased. If the transfer is from the positive to the negative point, condenser capacitance should be decreased. This doesn't apply to twins and other multi-cylinder magnetos because the direction of the primary current flow and therefore the points polarity alternates each time the points open. As a result, the contacts in these magnetos should remain fairly smooth with no pitting. Most condensers have a capacitance value between the values of 100-250nF. Note that condensers used for battery/coil ignition systems are usually twice these values because they need to cater for the, usually 12v, battery voltage already present across the points when they close. Also, it is worth mentioning that the battery/coil systems always have the primary current flowing in one direction so the notes given above re pitting and contact polarity can be used to 'fine tune' the capacitance value selected.
2: Working Voltage. This is the maximum voltage which can be safely applied to the capacitor. If voltages higher than this are applied, the dielectric will break down, normally resulting in leakage of current through the capacitor. Luckily, some capacitors with particular dielectrics and thin metal electrodes get round the problem because the metal melts or evaporates in a breakdown vicinity, isolating it from the rest of the capacitor. Voltages in the primary circuit of a magneto typically reach several hundred volts.
3: dV/dt rating. This means ‘rate of change in Voltage with respect to Time’. In simple terms this indicates how fast a capacitor can be safely charged and discharged. If the capacitor is charged and discharged at a rate faster than its dV/dt rating, it will heat up internally and quickly fail. Some capacitor data sheets do not provide a dV/dt rating. This should probably be taken as an indication that the capacitor’s performance under conditions involving rapidly changing voltages is not very good. When a dV/dt rating is given, it is usually expressed as a number of volts per microsecond. For a magneto application, we should be looking for a value of several hundred V/µs.
The capacitors used as condenser replacements by The Magneto Guys are
Evox Rifa PME271M metallized paper capacitors. These feature a high dielectric constant with excellent self-healing properties and transient handling capabilities. They are constructed of a multi-layer metallized paper dielectric encapsulated and impregnated in self-extinguishing material meeting the requirements of UL94V-O. Typical applications of the Evox Rifa PME271M capacitors are interference suppressors – class X2.
• Rated voltage: 275 VAC (In DC Applications Recommended voltage ≤ 630 VDC)
• 100% screening factory test at 2,150 VDC *
• Excellent self-healing properties ensure long life even when subjected to frequent over voltages **
• Good resistance to ionization due to impregnated dielectric
• High dV/dt capability (600 V/μs)
• The impregnated paper ensures excellent stability and outstanding reliability properties, especially in applications with continuous operation
• Insulation Resistance Minimum Values Between Terminals C ≤ 0.33 μF ≥ 12,000 MΩ
*It is interesting to note that the data sheet states that the factory carries out a 100% screening test by applying 2,150Vdc between the terminals before carrying out all other tests. They do not recommend that this test is repeated but it does indicate that there is likely to be quite some margin over the quoted rated voltage.
**The self healing properties of Evox Rifa Impregnated Paper Capacitors is described in the data sheet as follows: If a conducting particle or a voltage surge punctures the dielectric, an arc occurs at the point of failure melting the surrounding metal and insulating the area of the breakdown. In the metallized paper capacitor as processed by Evox Rifa a breakdown caused by a short transient normally results in an improvement of the insulation resistance. Evox Rifa metallized paper capacitors are to be preferred if the capacitor is to be used where uncontrolled transient voltages can occur.
Group was acquired by
KEMET Corporation in 2008. The PME271 range of capacitors we use can be found under either name.
Alternatives to the EVOX RIFA PME271M capacitors.
Yes, other alternatives are available which probably/possibly work just as well. We use the EVOX RIFA capacitors because they are readily available at a reasonable cost and, more importantly, experience over many years, with them fitted in thousands of magnetos, has proven that they work well - we have had no electrical failures with them at all. We know that other magneto restorers use the EVOX RIFAs too. We also know that some magneto restorers choose to use other types. We have no experience of these other types so we stick with what we know does work. We see no need to experiment with any new options which come on the market all the time the EVOX RIFAs are available.
How we test the original capacitors and, if necessary, fit the new replacements is described in our Workshop Pages here